pole mounted transformers SKM

[nsarnaik]

Hello everyone Merry Christmas!

I am trying to model an overhead system in SKM. I have some instances of three pole mounted single phase transformers. In the instance where they all have the same KVA rating, I have just modeled it as a 3-phase transformer. However, I have one instance where I have two 15KVA single phase transformers and one 50KVA single phase transformer.

Right now I have shown three single phase transformers coming off a pole (shown as a bus) and three feeders coming off the secondary side, each connected to a phase A, B and C. SKM doesn't like this and I'm getting the error that my voltage ratio of the bus on the primary side of the transformers to the bus on the secondary side is less than or greater than 10%. I could just model this as a 3 phase transformer but then how would I incorporate the unbalanced ratings of the transformers?

Thank you

 

[David Castor]

I haven't tried it in SKM, but it may not be possible to directly model this oddball configuration. I'd contact SKM tech support. They are generally pretty helpful (if you're on maintenance). If your main concern is short circuit current and/or arc-flash, I would model as three 50 kVA and again as three 15 kVA and take the worst case. If the secondary is 120/240 or 208/120 V, it probably isn't going to make a huge difference.

 

[JoeStillman]

I was able to do it by setting up three 3Ø transformers in parallel and then changing them to single phase.

 

[tortuga]

I have one instance where I have two 15KVA single phase transformers and one 50KVA single phase transformer.

Take a drive out to the pole and re-check could be the field survey is incorrect.
How is the bank connected?

 

[JoeStillman]

Take a drive out to the pole and re-check could be the field survey is incorrect.
How is the bank connected?

It sounds to me like a 4WD arrangement. The 50 kVA would be the one with the center tap.

 

[tortuga]

It sounds to me like a 4WD arrangement. The 50 kVA would be the one with the center tap.

I was told by an engineer once that 'closed' delta all have to be equal size transformers. The explanation went over my head and I don't remember it.

 

[JoeStillman]

I was told by an engineer once that 'closed' delta all have to be equal size transformers. The explanation went over my head and I don't remember it.

I've often seen unequal transformers in open 4WD. Can't remember the last time I saw a 4WD fed from three transformers though.

 

[Birken Vogt]

I can probably count on two hands the number of closed delta in my area. At least half of them have smaller wing pots.

Actually this picture is on my wall. Estimate about 1910.